Phosphor, process for preparation thereof and lamp containing the same

ABSTRACT

Disclosed is a phosphor emitting a green to bluish white fluorescence, which is represented by the following general formula: M3-xTbxBO6 wherein M stands for at least one element selected from the group consisting of La, Gd and Y, and x is a number of 0&lt;x&lt;/=2.0. This phosphor has a high brightness, and in a fluorescent lamp prepared by using this phosphor, degradation of the brightness is very small.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rate earth borate phosphor and aprocess for the preparation thereof, and the present invention alsorelates to a fluorescent screen comprising this phosphor.

2. Description of the Prior Art

As the known rare earth borate phosphor, there can be mentioned, forexample, a phosphor disclosed in Japanese Patent Application Laid-OpenSpecification No. 33986/78, which is represented by the followinggeneral formula:

    La.sub.1-x-y-z Gd.sub.x A'.sub.y A".sub.z B.sub.3 O.sub.6

wherein A' is Bi or Ce, A" is Tb or dy, x is a number of 0≦x≦1, y is anumber of 0.001≦y≦0.80 and z is a number of 0≦z≦0.60, with the provisothat x, y and z satisfy the requirement of x+y+z≦1.

The host lattice of this phosphor is composed of a lanthanoidsesquioxide Ln₂ O₃ (in which Ln is La or Gd) and B₂ O₃ at a molar ratioof 1:3 and the phosphor emits a peculiar fluorescence determined by theactivator A' and/or A". As another rare earth borate phosphor, there canbe mentioned Gd₂ O₃.B₂ O₃, La₂ O₃.B₂ O₃ activated with trivalenteuropium (Eu³⁺), which is disclosed J. Electrochem. Soc., 111, 12, page1363 (1964). The host lattice of the phosphor of this type is composedof a lanthanoid sesquioxide Ln₂ O₃ (in which Ln is Gd or La) and B₂ O₃at a molar ratio of 1:1 and the phosphor emits a red fluorescenceinherent of Eu³⁺.

These known phosphors, however, are still insufficient in the brightnessand the maintenance of fluorescent characteristics.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a novelphosphor having a high brightness, in which fluorescent characteristicscan be maintained for a long time and a process for the preparation ofthis novel phosphor.

This and other objects of the present invention can be attained by aphosphor represented by the following general formula:

    M.sub.3-x Tb.sub.x BO.sub.6

wherein M stands for at least one element selected from the groupconsisting of La, Gd and Y, and x is a number of 0≦x≦2.0.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relation between the Tb concentrationand the relative brightness, which is given to illustrate the presentinvention.

FIGS. 2 and 5 are diagrams showing the fluorescent spectra of thephosphor of the present invention and the conventional phosphor.

FIG. 3 is a diagram showing the X-ray diffraction patterns of thephosphor of the present invention and the conventional phosphor.

FIG. 4 is a diagram illustrating the exciting spectrum of the phosphorof the present invention.

FIG. 6 is a diagram showing the relation between the amount of the fluxand the relative emission intensity of the phosphor, which is given toillustrate the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In addition to the above-mentioned rare earth borate host latticeshaving a constituent molar ratio of 1:3 or 1:1, there is known a boratein which the constituent molar ratio is 3:1. For example, the presenceof 3La₂ O₃.B₂ O₃, that is, La₃ BO₆, is confirmed in J. American CeramicSoc., 44, 2, page 87 (1961). However, a borate phosphor having aconstituent molar ratio of 3:1 has not been reported.

We found that when a predetermined fluoride is used as the flux, therecan be obtained a Tb-activated phosphor represented by the above generalformula, which is composed of a lanthanoid sesquioxide and boricanhydride at a molar ratio of 3:1.

When Gd is used as the element M in the general formula, a phosphorhaving a highest brightness is obtained. Therefore, Gd is most preferredas M. When two or more of the elements are used as M, it is preferredthat Gd be contained in an amount of at least 80 mole %, especially atleast 90 mole %, based on M. The higher is the Gd content, the higher isthe brightness. When Y is used as M, the brightness is reduced. It ispreferred that Y be used in combination with other element so that the Ycontent is not higher than 5 mole %.

The value of x is limited within the above-mentioned range because thebrightness of the phosphor is high within this range. Even if the amountof Tb is very small, emission is observed, and when x is in the range of0.01≦x≦1.5, the brightness of the phosphor is significantly improved.Therefore, it is preferred that the value of x be adjusted within thisrange. When the value x is in the range of 0.1≦x≦0.8, the brightness ofthe phosphor is most improved. Accordingly, it is most preferred thatthe value x be adjusted within the above range.

The phosphor of the present invention can be obtained by firing oxidesof rare earth elements and/or compounds of rare earth elements that canbe converted to said oxides by firing, boric acid and/or boricanhydride, and at least one fluoride selected from the group consistingof fluorides represented by the following general formula:

    M'BF.sub.4

wherein M' stands for at least one element or atomic group selected fromthe group consisting of Li, K, Na and NH₄,

fluorides represented by the following general formula:

    M'.sub.2 SiF.sub.6

wherein M' is as defined above,

and fluorides represented by the following general formula:

    M"SiF.sub.6

wherein M" stands for at least one element selected from the groupconsisting of Zn, Mg, Pb, Ca, Sr and Ba.

It is preferred that boric acid and/or boric anhydride be incorporatedin such an amount that the amount of boron is 1.0 to 1.5 times thestoichiometric amount of boron in the phosphor represented by the abovegeneral formula. If the amount of boron exceeds 1.5 times thestoichiometric amount, the brightness of the obtained phosphor isreduced. In order to obtain a phosphor having a highly improvedbrightness, it is preferred that the amount of boric acid and/or boricanhydride be determined so that the amount of boron is 1.0 to 1.05 timesthe stoichiometric amount.

It is preferred that the fluoride acting as the flux be incorporated inan amount of 2 to 50% by weight, especially 5 to 50% by weight, based onthe starting materials of the phosphor. If the amount of the fluoride issmaller than 2% by weight, the fluoride exerts no substantial activityas the flux.

Even if the fluoride is incorporated in an amount exceeding 50% byweight, a phosphor can be prepared. In this case, however, a long timeis necessary for removing the flux from the prepared phospor, and use ofa large quantity of the flux is not preferred from the economicalviewpoint. Therefore, it is preferred that the amount of the flux beadjusted within the abovementioned range.

It is preferred that firing be carried out after prefiring has beenconducted in air at a temperature from 700° to 1000° C. for 2 to 5hours. When compounds other than oxides are used as the startingmaterials of the phosphor and they are converted to oxides, the processof the prefiring is indispensable. Furthermore, it is preferred thatfiring be conducted 2 or 3 times rather than 1 time (these firings willbe referred to as "first firing", "second firing" and "third firing"hereinafter). It is preferred that each firing be conducted at atemperature of 1000° to 1300° C. in an inert gas, nitrogen or a weaklyreducing atmosphere for 2 to 10 hours. It is especially preferred thatfiring be conducted at a temperature of 1150° to 1250° C. for 3 to 10hours, because the brightness is improved and degradation of thebrightness at the time of baking in air is reduced. The weakly reducingatmosphere may be formed by mixing an inert gas such as N₂ with a smallamount of a reducing gas such as hydrogen, carbon dioxide gas or carbonmonoxide gas. It is preferred that the amount of the reducing gas to bemixed be 3 to 20% by volume, especially 5 to 10% by volume. It ispreferred that before each of first or second or third firing, thesample be once cooled, pulverized and re-mixed.

The flux may be incorporated before prefiring or before first, second orthird firing. If firing is carried out at least one time afterincorporation of the flux, the brightness can be greatly improved.

The phosphor of the present invention may be used without leaching.However it is better to use after water washing and drying. It ispreferred that the phosphor be used after the fired product ispulverized, treated with a 0.1 to 1 N weak acid, washed with watersufficiently and dried. By this method, the brightness of the phosphorscan be highly improved. As the acid, there is used an aqueous solutionof HCl, HNO₃ or H₂ SO₄.

In the phosphor of the present invention, no substantial degradation ofthe brightness is caused at the step of baking in air for use in afluorescent lamp. Moreover, the lumen maintenance of a fluorescent lampcoated with the phosphor of the present invention is much improvedcompared with the halophosphate lamps. Accordingly, if the phosphor ofthe present invention is used singly or in combination with otherphosphors, an excellent fluorescent lamp can be obtained.

The present invention will now be described in detail with reference tothe following examples.

EXAMPLES 1 THROUGH 6

In order to obtain phosphors of the general formula shown in Table 1,starting materials except (NH₄)₂ SiF₆ as the flux were taken at molarratios shown in Table 1 and sufficiently mixed. Each mixture wasprefired at a temperature of 700° C. for 2 hours, and the fired productwas cooled, pulverized and re-mixed. The prefired mixture were thensubjected first firing. In case of Example 2, after (NH₄)₂ SiF₆ had beenadded in an amount shown in Table 1 (% by weight based on the startingmaterials of the phosphor; the same will hold good hereinafter), firstfiring was carried out. In Examples 3 through 6, the fired mixtures werecooled, pulverized, mixed and subjected to second firing. In Example 4and 6, (NH₄)₂ SiF₆ was added before second firing. That is, in theseExamples, the flux was added before final firing. In Table 1, H₂ /N₂means a weakly reducing atmosphere comprising 5% by volume of H₂ and 95%by volume of N₂ (the same will hold good hereinafter).

The products passed through 300 mesh screen were Y₂.8 Tb₀.2 BO₆ inExamples 1 and 2, phosphors La₂.8 Tb₀.2 BO₆ in Examples 3 and 4 andphosphors Gd₂.8 Tb₀.2 BO₆ in Examples 5 and 6.

Each phosphor emits a green fluorescence when excited by 254 nm mercuryvapor resonance rays. The relative brightness of the samples obtained bythe above procedure are shown in Table 1. The upper value of therelative brightness is a value calculated based on the supposition thatthe brightness of Zn₂ SiO₄ :Mn (NBS-1021, one of the standard phosphorsamples issued by The National Bureau of Standards, Washington D.C.,U.S.A.) is 100%, and the lower value of the relative brightness is avalue calculated based on the supposition that the brightness of theflux-free phosphor (the phosphors of Examples 1, 3 and 5 for thephosphors of Examples 2, 4 and 6, respectively) is 100%.

                  TABLE 1                                                         ______________________________________                                                Ex-  Ex-    Ex-      Ex-  Ex-    Ex-                                          am-  am-    am-      am-  am-    am-                                          ple  ple    ple      ple  ple    ple                                          1    2      3        4    5      6                                    ______________________________________                                        General For-                                                                  mula                                                                          M         Y      Y      La     La   Gd     Gd                                 x         0.2    0.2    0.2    0.2  0.2    0.2                                Starting                                                                      Materials                                                                     Y.sub.2 O.sub.3                                                                         1.4    1.4    --     --   --     --                                 La.sub.2 O.sub.3                                                                        --     --     1.4    1.4  --     --                                 Gd.sub.2 O.sub.3                                                                        --     --     --     --   1.4    1.4                                Tb.sub.4 O.sub.7                                                                        0.05   0.05   0.05   0.05 0.05   0.05                               H.sub.3 BO.sub.3                                                                        1.05   1.05   1.05   1.05 1.05   1.05                               (NH.sub.4).sub.2 SiF.sub.6                                                              0      20     0      20   0      20                                 (% by weight)                                                                 Firing                                                                        Conditions                                                                    Preliminary                                                                             700° C., 2 hours in air                                      Firing                                                                        First     1200° C.,                                                                        1200° C.,                                          Firing    2 hours   2 hours                                                             in H.sub.2 N.sub.2                                                                      in nitrogen                                               Second    --     --     1200° C.,                                                                        1200° C.,                            Firing                  2 hours   2 hours                                                             in nitrogen                                                                             in H.sub.2 /N.sub.2                         Relative                                                                      Brightness (%)                                                                to Zn.sub.2 SiO.sub.4 :Mn                                                               5.4    39.4   3.1    74.6 2.3    103.0                              to phosphor                                                                   free of                                                                       (NH.sub.4).sub.2 SiF.sub.6                                                              100    730    100    2410 100    4480                               ______________________________________                                    

EXAMPLE 7 THROUGH 16

Phosphors Gd_(3-x) Tb_(x) BO₆ were prepared by using starting materialsshown in Table 2 in the same manner as described in Example 6.Incidentally, (NH₄)₂ SiF₆ was added before second firing. The relativebrightness of each phosphor (to Zn₂ SiO₄ :Mn) is shown in Table 2. Theupper value of the relative brightness is a value of a product obtainedonly by pulverizing the obtained phosphor, and the lower value of therelative brightness is a value of a product obtained by pulverizing theobtained phosphor, washing the pulverized phosphor with 0.6 N HCl,washing the treated phosphor sufficiently with water and drying thewashed phosphor. The relation between the Tb concentration and therelative brightness of the hydrochloric acid-treated phosphor is shownby curve 1 in FIG. 1. As is apparent from FIG. 1, a highest brightnesswas obtained when x was about 0.4.

In order to examine the brightness maintenance ratio after baking inair, the hydrochloric acid-treated phosphor Gd₂.6 Tb₀.4 BO₆ in which xwas 0.4 was re-fired at 620° C. for 10 minutes in air. The ratio of thebrightness after re-firing to the brightness before re-firing that is,the brightness maintenance ratio after baking in air, was 98.9%. Andalso there was no temperature quenching of the brightness even attemperatures of 150° C. or greater. Then the phosphors of the presentinvention can be utilized for a high-loaded low pressure typefluorescent lamp having an operated temperature range of approximately150° C. or greater.

                                      TABLE 2                                     __________________________________________________________________________            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                          7    8    9    10   11   12   13   14   15   16                       __________________________________________________________________________    General                                                                       Formula                                                                       M       Gd   Gd   Gd   Gd   Gd   Gd   Gd   Gd   Gd   Gd                       x       0.01 0.05 0.10 0.20 0.40 0.50 0.60 0.80 1.20 1.50                     Starting                                                                      Materials                                                                     Gd.sub.2 O.sub.3                                                                      1.495                                                                              1.475                                                                              1.45 1.40 1.30 1.25 1.20 1.10 0.90 0.75                     Tb.sub.4 O.sub.7                                                                      0.0025                                                                             0.0125                                                                             0.025                                                                              0.05 0.1  0.125                                                                              0.15 0.2  0.3  0.375                    H.sub.3 BO.sub.3                                                                      1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05                     (NH.sub.4).sub.2 SiF.sub.6                                                            20   20   20   20   20   20   20   20   20   20                       (% by weight)                                                                 Firing                                                                        Conditions                                                                    Preliminary                                                                   Firing        700° C., 2 hours in air                                  First                                                                         Firing       1200° C., 2 hours in nitrogen                             Second                                                                        Firing       1200° C., 2 hours in nitrogen                             Relative                                                                       Brightness (%)                                                               Pulverized                                                                    Phosphor                                                                              30.4 64.2 75.8 90.1 116.5                                                                              110.2                                                                              --   91.4 38.3 21.4                     HCl-Treated                                                                   Phosphor                                                                              32.5 70.0 91.4 102.1                                                                              121.1                                                                              --   119.4                                                                              91.4 44.5 25.1                     __________________________________________________________________________

EXAMPLES 17 THROUGH 26

Phosphors of the general formula shown in Table 3, that is, the samephosphors Gd_(3-x) Tb_(x) BO₆ as those of Examples 7 through 16, wereprepared in the same manner as in Examples 7 through 16 except thatsecond firing was carried out at 1200° C. for 2 hours in H₂ /N₂. Sincethe amounts of the starting materials corresponding to the value x werethe same as those in Examples 7 through 16, they are not shown in Table3. The relative brightness of each phosphor is shown in Table 3. Theupper and lower values are as defined in Table 2. The relativebrightness of the HCl-treated phosphor is indicated by curve 2 inFIG. 1. It is seen that the effect of firing in the weakly reducingatmosphere was especially prominent when the Tb concentration was high.

The emission spectra of the obtained phosphors excited by mercury vaporresonance rays are shown in FIG. 2. Curve 4 indicates the emissionspectrum of NBS-1021, Zn₂ SiO₄ :Mn, curve 5 indicates the emissionspectrum of the phosphor obtained in Example 21 and curve 6 indicatesthe emission spectrum of the phosphor obtained in Example 17. Thephosphors having a high Tb concentration emitted a green fluorescence,and the phosphors having a low Tb concentration emitted a bluish whitefluorescence.

The uppor portion (3-1) of FIG. 3 shows the X-ray diffraction pattern ofthe known phosphor (Gd₀.7 Ce₀.2 Tb₀.1)₂ O₃. 3B₂ O₃ composed of rareearth sesquioxides and B₂ O₃ at a molar ratio of 1:3, and the lowerportion (3-2) of FIG. 3 shown the X-ray diffraction pattern of thephosphor obtained in Example 10. Both the patterns are apparentlydifferent from each other.

The excited spectrum of the acid-treated phosphor obtained in Example 24is shown in FIG. 4.

                  TABLE 3                                                         ______________________________________                                                        Relative Brightness (%)                                       Example  General Formula                                                                            after pulveri-                                                                           after HCl                                    No.      M      x         zation   treatment                                  ______________________________________                                        17       Gd     0.01      32.5     36.2                                       18       Gd     0.05      64.8     76.9                                       19       Gd     0.10      75.6     90.1                                       20       Gd     0.20      88.6     103.0                                      21       Gd     0.40      108.6    124.4                                      22       Gd     0.50      --       127.2                                      23       Gd     0.60      --       129.1                                      24       Gd     0.80      96.2     111.8                                      25       Gd     1.20      51.5     60.4                                       26       Gd     1.50      31.0     37.5                                       ______________________________________                                    

EXAMPLES 27 THROUGH 36

Phosphors La_(3-x) Tb_(x) BO₆ were prepared by using the startingmaterials shown in Table 4 in the same manner as described in Examples 3and 4 under conditions shown in Table 4. Incidentally, (NH₄)₂ SiF₆ wasadded before second firing. The relative brightness of each phosphor toZn₂ SiO₄ :Mn is shown in Table 4. The left value is a value of thepulverized phosphor, the middle value is a value of the pulverized,water-washed and dried phosphor, and the right value is a value of theproduct obtained by pulverizing the phosphor, washing the pulverizedphosphor with 0.6 N HCl, washing the treated phosphor sufficiently withwater and drying the washed phosphor. The relation between the Tbconcentration and the relative brightness of the hydrochloricacid-treated phosphor is indicated by curve 3 in FIG. 1. From thiscurve, it is seen that the highest relative brightness was obtained whenx was about 0.8.

The emission spectrum of the phosphor obtained in Example 33 whenexcited with mercury vapor resonance rays is shown by curve 7 in FIG. 5.Incidentally, curve 4 indicates the emission spectrum of NBS-1021, Zn₂SiO₄ :Mn.

                                      TABLE 4                                     __________________________________________________________________________    Charging Conditions                                                           General    Starting Materials  Firing Conditions                                                                            Relative Brightness (%)         Example                                                                             Formula          (NH.sub.4).sub.2 SiF.sub.6                                                            First                                                                              Second                                                                             Third                                                                              Pulveri-                                                                           Water                                                                              HCl                   No.   M x  La.sub.2 O.sub.3                                                                  Tb.sub.4 O.sub.7                                                                  H.sub.3 BO.sub.4                                                                  (% by weight)                                                                         Firing                                                                             Firing                                                                             Firing                                                                             zation                                                                             Washing                                                                            Treatments            __________________________________________________________________________    27    La                                                                              0.01                                                                             1.495                                                                             0.0025                                                                            1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    6.9 --   7.6                                                  2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  28    La                                                                              0.05                                                                             1.475                                                                             0.0125                                                                            1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   31.4 --   34.1                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  29    La                                                                              0.10                                                                             1.45                                                                              0.025                                                                             1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   47.2 --   53.5                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  30    La                                                                              0.20                                                                             1.4 0.05                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   68.2 72.4 74.6                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  31    La                                                                              0.40                                                                             1.3 0.10                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   84.1 --   88.5                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  32    La                                                                              0.60                                                                             1.2 0.15                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   90.3 93.2 96.8                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  33    La                                                                              0.80                                                                             1.1 0.20                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   99.0 --   101.5                                                2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  34    La                                                                              1.00                                                                             1.0 0.25                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   86.5 --   94.1                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  35    La                                                                              1.20                                                                             0.9 0.30                                                                              1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   63.1 --   69.4                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  36    La                                                                              1.50                                                                             0.75                                                                              0.375                                                                             1.05                                                                              20      700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                   66.0 --   67.9                                                 2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitro-                                                                gen  gen                                  __________________________________________________________________________

EXAMPLES 37 THROUGH 46

Phosphors Gd₂.5 Tb₀.5 BO₆ (Examples 37 through 41), La₂.8 Tb₀.2 BO₆(Examples 42 and 43) and Gd₂.2 Tb₀.8 BO₆ (Examples 44, 45 and 46) wereprepared by using the starting materials shown in Table 5 underconditions shown in Table 5 in the same manner as in Examples 3 through6. Incidentally, (NH₄)₂ SiF₆ was added as the flux before second firing.The relative brightness of each phosphor is shown in Table 5. The leftvalue is a relative value to Zn₂ SiO₄ :Mn and the right value is arelative value to the flux-free phosphor.

The relation between the amount of the flux and the relative emissionintensity of the obtained phosphor in Examples 37 through 41 is shown inFIG. 6, from which it is seen that a highest brightness was obtainedwhen the amount incorporated of the flux was about 20% by weight.

    __________________________________________________________________________    Charging Conditions                                                                     Starting Materials       Firing Conditions                                                                             Relative Brightness                                                           (%)                        Ex- General                  (NH.sub.4).sub.2 SiF.sub.6                                                          Preli-                 To                  ample                                                                             Formula                  (% by minary                                                                             First                                                                              Second                                                                              To     flux-free           No. M  x  La.sub.2 O.sub.3                                                                  Gd.sub.2 O.sub.3                                                                  Tb.sub.4 O.sub.7                                                                  H.sub.3 BO.sub.3                                                                  B.sub.2 O.sub.3                                                                  Weight)                                                                             Firing                                                                             Firing                                                                             Firing                                                                              Zn.sub.2 SiO.sub.4                                                            :Mn    phosphor            __________________________________________________________________________    37  Gd 0.5                                                                              --  1.25                                                                              0.125                                                                             1.05                                                                              --  0    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    4.5    100                                                    2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   38  Gd 0.5                                                                              --  1.25                                                                              0.125                                                                             1.05                                                                              --  2    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    33.0   731                                                    2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   39  Gd 0.5                                                                              --  1.25                                                                              0.125                                                                             1.05                                                                              --  5    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    41.1   913                                                    2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   40  Gd 0.5                                                                              --  1.25                                                                              0.125                                                                             1.05                                                                              -- 20    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    127.2  2820                                                   2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   41  Gd 0.5                                                                              --  1.25                                                                              0.125                                                                             1.05                                                                              -- 50    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    36.6   813                                                    2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   42  La 0.2                                                                              1.4 --  0.05                                                                              1.05                                                                              -- 20    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    62.8   --                                                     2 hours.                                                                           2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitrogen                                                              gen                                   43  La 0.2                                                                              1.4 --  0.05                                                                              --  1.05                                                                             20    700° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    67.5   --                                                     2 hours                                                                            2 hours                                                                            2 hours                                                             in air                                                                             in nitro-                                                                          in nitrogen                                                              gen                                   44  Gd 0.80                                                                             --  1.10                                                                              0.20                                                                              1.05                                                                              -- 20    900° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    102.4  --                                                     2 hours                                                                            2 hours                                                                            5 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   45  Gd 0.80                                                                             --  1.10                                                                              0.20                                                                              1.10                                                                              -- 20    900° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    101.0  --                                                     2 hours                                                                            2 hours                                                                            5 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   46  Gd 0.80                                                                             --  1.10                                                                              0.20                                                                              1.20                                                                              -- 20    900° C.,                                                                    1200° C.,                                                                   1200° C.,                                                                    89.7   --                                                     2 hours                                                                            2 hours                                                                            5 hours                                                             in air                                                                             in nitro-                                                                          in H.sub.2 /N.sub.2                                                      gen                                   __________________________________________________________________________

EXAMPLES 47 THROUGH 52

Phosphors La₀.26 Gd₂.34 Tb₀.4 BO₆ (Examples 47 and 48) and Gd₂.5 Tb₀.5BO₆ (Examples 49 through 52) were prepared by using the startingmaterials shown in Table 6 under conditions shown in Table 6 in the samemanner as in Examples 3 through 6. Incidentally, the flux was addedbefore second firing It was found that the relative brightness wasincreased to 2160%, 1310% and 913% by addition of Na₂ SiF₆, ZnSiF₆ andNH₄ BF₄ as the flux, independently.

                  TABLE 6                                                         ______________________________________                                                 Ex-   Ex-     Ex-    Ex-  Ex-   Ex-                                           am-   am-     am-    am-  am-   am-                                           ple   ple     ple    ple  ple   ple                                           47    48      49     50   51    52                                   ______________________________________                                        General                                                                       Formula                                                                       M          La,Gd   La,Gd   Gd   Gd   Gd    Gd                                 x          0.4     0.4     0.5  0.5  0.5   0.5                                Starting                                                                      Materials                                                                     La.sub.2 O.sub.3                                                                         0.13    0.13    --   --   --    --                                 Gd.sub.2 O.sub.3                                                                         1.17    1.17    1.25 1.25 1.25  1.25                               Tb.sub.4 O.sub.7                                                                         0.10    0.10    0.125                                                                              0.125                                                                              0.125 0.125                              H.sub.3 BO.sub.3                                                                         1.05    1.05    1.05 1.05 1.05  1.05                               (NH.sub.4).sub.2 SiF.sub.6                                                               0       20      0    0    0     0                                  (% by weight)                                                                 Na.sub.2 SiF.sub.6                                                                       0       0       0    20   0     0                                  (% by weight)                                                                 ZnSiF.sub.6                                                                              0       0       0    0    20    0                                  NH.sub.4 BF.sub.4                                                                        0       0       0    0    0     20                                 Firing                                                                        Conditions                                                                    Preliminary                                                                               700° C., 2 hours in air                                    Firing                                                                        First      1200° C., 2 hours in nitrogen                               Firing                                                                        Second     1200° C., 2 hours in H.sub.2 /N.sub.2                       Firing                                                                        Relative                                                                      Brightness (%)                                                                To Zn.sub.2 SiO.sub.4 :Mn                                                                20.1    112.5   5.5  97.5 59.0  41.1                               To Flux-Free                                                                             100     559     100  2160 1310  913                                Phosphor                                                                      ______________________________________                                    

What is claimed is:
 1. A phosphor represented by the following generalformula:

    M.sub.3-x Tb.sub.x BO.sub.6

wherein M stands for at least one element selected from the groupconsisting of La, Gd and Y, and x is a number of 0.1≦x≦0.8.
 2. Aphosphor as set forth in claim 1, wherein M in the general formula isGd.
 3. A phosphor as set forth in claim 1, wherein M in the generalformula is Gd and at least one other element selected from said groupand Gd occupies at least 80 mole % of M.
 4. A phosphor as set forth inclaim 1, wherein M in the general formula is La.
 5. A phosphor as setforth in claim 1, wherein M in the general formula is Y.
 6. A phosphoras set forth in claim 1, wherein M in the general formula is Y and atleast one other element, selected from said group and Y occupies at most5 mol % of M.
 7. A fluorescent lamp having incorporated therein aphosphor according to claim
 1. 8. A process for the preparation of aphosphor represented by the following general formula:

    M.sub.3-x Tb.sub.x BO.sub.6

wherein M stands for at least one element selected from the groupconsisting of La, Gd and Y, and x is a number of 0.1≦x≦0.8,whichcomprises heating an oxide of at least one rare earth element selectedfrom the group consisting of La, Gd and Y or a rare earth metal compoundcapable of being converted to said oxide, an oxide of Tb or a Tbcompound capable of being converted to the oxide of Tb by firing and atleast one boron compound selected from the group consisting of boricacid and boric anhydride, wherein said heating comprises firing at 1000°C.-1300° C. in an inert gas or weakly reducing gas, and in the presenceof at least one fluoride selected from the group consisting of fluoridesrepresented by the following general formula:

    M'BF.sub.4

wherein M' stands for at least one element or atomic group selected fromthe group consisting of Li, K, Na and NH₄,fluorides represented by thefollowing general formula:

    M'.sub.2 SiF.sub.6

wherein M' is as defined above,and fluorides represented by thefollowing general formula:

    M"SiF.sub.6

wherein M" stands for at least one element selected from the groupconsisting of Zn, Mg, Pb, Ca, Sr and Ba.
 9. A process according to claim8, wherein the amount of the fluoride is 2 to 50% by weight based on thestarting materials of the phosphor.
 10. A process according to claim 9,wherein the amount of the fluoride is 5 to 50% by weight based on thestarting materials of the phosphor.
 11. A process according to claim 8,wherein the boron compound is incorporated in such an amount that theamount of boron in the phosphor is 1.0 to 1.05 times the stoichiometricamount.
 12. A process according to claim 8, wherein said heating furthercomprises pre-firing in air at 700 to 1000° C. prior to firing in aninert gas or weakly reducing gas at 1000° to 1300° C.
 13. A processaccording to claim 8, wherein the product obtained by said heating istreated with a weak acid, washed with water and then dried.
 14. Aprocess according to claim 12, wherein the weak acid is a weak acidhaving a concentration of 0.1 to 1 Normal.
 15. A process according toclaim 13 or 14, wherein the weak acid is an aqueous solution of HCl,HNO₃ or H₂ SO₄.
 16. A process according to claim 8, wherein M in thegeneral formula is Gd.
 17. A process according to claim 8, wherein M inthe general formula is Gd and at least one other element selected fromsaid group and Gd occupies at least 80 mole % of M.
 18. A processaccording to claim 8 or 12, wherein said firing is conducted for 2-10hours.
 19. A process according to claim 8 or 12, wherein said firing inthe inert or weakly reducing gas is conducted two or three times, andwherein at least one of the firing steps is carried out in the presenceof said at least one fluoride.
 20. A process according to claim 12,wherein said pre-firing is conducted for 2-5 hours.
 21. A processaccording to claim 8 or 12, wherein said firing in the inert or weaklyreducing gas is conducted at a temperature of 1150°-1250° C. for 3-10hours.
 22. A process according to claim 12, wherein said pre-firing iscarried out in the presence of said at least one fluoride.
 23. A processaccording to claim 8 or 12, wherein said firing is carried out in aweakly reducing gas, said weakly reducing gas being a mixture of aninert gas and 3-20% by volume of a reducing gas selected from the groupconsisting of hydrogen, carbon dioxide and carbon monoxide.